The present invention relates to the use of nanoscale metal oxide particles as polymerization catalysts. Particularly, the present invention relates to the use of nanoscale metal oxide particles as catalysts which can replace the conventional catalysts for the thermal and photochemical polymerization of (e.g. radically) polymerizable species such as, e.g., peroxides, azo compounds and the conventional UV polymerization initiators. This allows the manufacture of inorganic-organic composite materials containing or consisting of an inorganic network which does not contain any residues derived from said conventional polymerization initiators.
It is already known to crosslink silicon containing polycondensates or heteropoly-condensates wherein, for example, an epoxy group or a methacrylic group is covalently bonded to a silicon atom in the presence of thermally or photochemically active catalysts by means of said functional organic groups. Moreover, it is known that by using nanoscale fillers which are homogeneously dispersed in an inorganic-organic matrix transparent molded articles and coatings may be produced.
According to the present invention it has sursprisingly been found that it is possible to effect a polymerization or crosslinking by means of certain polymerizable groupings even without the above-mentioned conventional catalysts if nanoscale particles (in the following sometimes referred to as nanoparticles) of certain substances are mixed with (e.g. dispersed in) said species which are to be polymerized or crosslinked, respectively and which show said polymerizable groupings, and the resulting mixture is treated thermally and/or irradiated (with UV light). This makes it possible, for example, to thermally and/or photochemically effect the polymerization of species (monomers, oligomers and polymers including polycondensates) having (meth)acrylate groups, as well as the polyaddition of species having an epoxide ring in the sole presence of said nanoparticles as catalysts. It is believed that the catalytic action of said nanoparticles is primarily due to the presence of (numerous) Lewis acid or Lewis base centers, respectively, on the surface thereof.
Although it is known that, e.g., aluminum alkyls catalyze polymerization reactions of double bonds according to the Ziegler-Natta process, similar catalytic effects of particles have not as yet been known.
Accordingly the present invention provides a process for the thermal and/or photochemical polymerization or crosslinking of species (monomers, oligomers and polymers including polycondensates) having at least one polymerizable carbon-carbon multiple bond and/or at least one carbon containing ring capable of undergoing a ring opening polymerization, said ring preferably containing at least one heteroatom selected from the group consisting of oxygen, nitrogen and sulfur as ring atom, wherein said process is characterized in that as (preferably sole) thermal and/or photochemical polymerization catalyst nanoscale particles of at least one metal oxide (including mixed oxides of metals) is used.
Said process makes it possible to produce, e.g., (highly transparent) molded articles and coatings, particularly for optical purposes, said molded articles and coatings being also an object of the present invention.
Among the advantages of the present invention is the fact that the conventional polymerization and polyaddition catalysts may be dispensed with and as a result thereof no corresponding decomposition products are present in the final polymer (e.g. molded article or coating) and that the polymerization catalysts employed according to the present invention are not subject to inhibition by oxygen, which constitutes a problem with many of the conventionally used (particularly UV) initiators.
In the following the process according to the present invention will be explained in more detail.
The species to be polymerized or crosslinked may be both purely organic species and mixed inorganic-organic species.
In the present description and the appended claims the term xe2x80x9corganic speciesxe2x80x9d is meant to denote species which in addition to carbon and hydrogen as mandatory components may optionally contain only elements selected from the group consisting of oxygen, nitrogen, sulfur and halogen (i.e., fluorine, chlorine, bromine and iodine). On the other hand, xe2x80x9cinorganic-organic speciesxe2x80x9d is to denote those species which in addition to the just mentioned elements may optionally contain further elements, particularly and preferably silicon, but also other elements such as, e.g., metals like aluminum, titanium and zirconium (preferably in addition to silicon).
According to the present invention preferred inorganic-organic species are (monomeric) hydrolyzable silicon compounds which in addition to one or more hydrolyzable groups (e.g. alkoxy groups) contain at least one non-hydrolyzable radical having a polymerizable carbon-carbon multiple bond (preferably a double bond) or a carbon containing ring capable of undergoing a ring opening polymerization (polyaddition) (preferably an epoxide ring) as well as precondensates (oligomers) and polycondensates derived from said monomeric silanes. Said precondensates or polycondensates, respectively, may in turn be those which are derived from one or more of the just described hydrolyzable silanes having a polymerizable carbon-carbon double bond or a ring capable of undergoing a ring opening polymerization as well as optionally, in addition thereto, from one or more other hydrolyzable silanes (without the just mentioned groups) and one or more hydrolyzable compounds of other elements cocondensable with said hydrolyzable silanes, for example those of aluminum, titanium and zirconium. It is, however, preferred that such precondensates and polycondensates are derived exclusively from hydrolyzable silanes.
The hydrolyzable silanes having a polymerizable group (in the following the term xe2x80x9cpolymerizable groupxe2x80x9d is meant to comprise not only polymerizable carbon-carbon multiple bonds but also carbon containing rings capable of undergoing a ring opening polymerization) are preferably compounds having 2 or 3, preferably 3, hydrolyzable radicals and 1 or 2, preferably 1, non-hydrolyzable radicals featuring a polymerizable group (preferably (meth)acrylate group or epoxide ring). Examples of hydrolyzable radicals are halogen (F, Cl, Br and 1, particularly Cl and Br), alkoxy (particularly C1-4 alkoxy such as, e.g., methoxy, ethoxy, n-propoxy, i-propoxy and butoxy), aryloxy (particularly C6-10 aryloxy such as phenoxy), acyloxy (particularly C1-4 acyloxy such as acetoxy and propionyloxy) and alkylcarbonyl (e.g. acetyl). Particularly preferred hydrolyzable radicals are alkoxy groups, especially methoxy and ethoxy.
Said polymerizable groups are bonded to the silicon atom preferably in the form of a group Rxe2x80x94Oxe2x80x94(CH2)nxe2x80x94Si, wherein R represents the group comprising the polymerizable entity and n has a value of from 1 to 10, preferably from 2 to 6. A particularly preferred linking group between R and Si is the oxypropyl group.
Hydrolyzable silicon compounds featuring a polymerizable group which are particularly preferred according to the present invention are those of the general formula
X3SiRxe2x80x2
wherein the groups X, the same or different from each other (preferably the same), represent a hydrolyzable group (preferably C1-4 alkoxy and particularly methoxy and ethoxy) and Rxe2x80x2 represents a glycidyloxy C1-6 alkylene or methacryloxy C1-6 alkylene radical.
It also is possible that in the above formula one or two radicals X, preferably one radical X, is replaced by a non-hydrolyzable radical without polymerizable group such as, e.g., an alkyl or aryl group, for example methyl, ethyl and phenyl.
Additional examples of hydrolyzable silanes having a polymerizable group are, e.g., those having a vinyl or allyl group directly bonded to the silicon.
Specific examples of hydrolyzable silanes to be employed as species to be polymerized or crosslinked, respectively (or as starting materials therefor) according to the present invention (optionally in the form of precondensates or polycondensates, respectively) are 3-glycidyloxypropyltrimethoxy silane, 3-glycidyloxypropyltriethoxy silane, 3-glycidyloxypropylmethyldimethoxy silane, 3-glycidyloxypropylmethyldiethoxy silane, 3-glycidyloxypropylethoxydimethoxy silane, 3-methacryloxypropyltrimethoxy silane, 3-methacryloxypropyltriethoxy silane, 3-methacryloxypropylmethyidichloro silane, 3-methacryloxypropylmethyldiethoxy silane and 3-methacryloxypropylmethyl-dimethoxy silane.
Among the purely organic species which can be polymerized according to the process of the present invention, preferred are those having at least one polymerizable carbon-carbon double bond (preferably activated by at least one electron-withdrawing group contained therein) and/or at least one carbon containing ring capable of undergoing a ring opening polymerization and featuring at least one heteroatom selected from the group consisting of O, S and N, and having 3 or 4 ring members (particularly oxirane, aziridine and oxetane ring). Specific examples of corresponding compounds are compounds derived from acrylic acid and methacrylic acid such as, e.g., the acids themselves, (meth)acrylonitrile, esters (preferably C1-4 alkyl esters), amides and anhydrides of said acids, maleic acid, maleic anhydride, fumaric acid, vinylacetate, vinylchloride, crotonic acid and derivatives thereof (e.g. esters and amides), styrene and derivatives thereof (particularly those having electron-withdrawing groups on the aromatic ring such as chlorostyrene), ethylene oxide, propylene oxide, butylene oxide, cyclopentene oxide, cyclohexene oxide, aziridine and oxetane and compounds containing groups corresponding to said ring compounds.
According to the present invention it is, of course, also possible to subject to polymerization or polyaddition, respectively other ring compounds, e.g., lactams like e-captolactam, such as lactams having 5 to 7 ring members.
The catalysts employed according to the present invention are metal oxides, preferably (mixed) oxides (including hydrated forms thereof) of metals from the main groups IIIa and IVa as well as the subgroups Ib, IIb, IVb, Vb and VIb of the Periodic Table of Elements.
Preferred according to the present invention are metal oxides of aluminum, tin, copper, silver, zinc, titanium, zirconium, vanadium, niobium, chromium, molybdenum and tungsten, the oxides of aluminum (particularly, boehmit), tin, titanium and zirconium being particularly preferred.
As already mentioned above said metal oxide particles are nanoscale particles. In the present description and the appended claims the term xe2x80x9cnanoscale particlesxe2x80x9d is meant to denote particles having an average particle size of not more than 200 nm, preferably not more than 100 nm, the particularly preferred particle size ranging from 2 to 50 nm. It is of course possible to also employ particles having a size of 1 nm and less, although said particles are less preferred due to their poorer availability.
Said nanoscale particles may be employed either as such (e.g. in powder form) or in the form of a (preferably aqueous and/or alcoholic) suspension, or may as well be prepared in situ.
The nanoscale metal oxide particles may be both amorphous and crystalline and may furthermore have optionally been subjected to a surface modification, for example by reacting a part of the OH groups present on the surfaces thereof with (preferably purely organic) compounds having, in addition to a group reactive with said OH groups (e.g. a carboxyl group), a polymerizable group identical with the polymerizable group of the species to be polymerized or crosslinked, respectively so that the catalyst particles are not only trapped in the resulting network but are even bonded thereto through covalent bonds. The surface modification of said nanoscale particles may, for example, be effected in a manner already described in detail in the prior art for the case of silica and alumina particles.
The preparation of the nanoscale particles may, for example, be carried out by hydrolyzing (preferably at room temperature) one (or more) hydrolyzable metal compound(s) such as a salt, a complex or an alkoxide (particularly C1-6 alkoxide, e.g., with methoxy, ethoxy, n-propoxy, i-propoxy, n-butoxy and butoxyethoxy as alkoxy group) in alcoholiclaqueous solutions. Said alcoholic/aqueous solutions preferably contain C1-6 alcohols (e.g. methanol, ethanol, n-propanol, i-propanol, n-butanol and/or butoxyethanol) and diluted inorganic acids (e.g. HCl solutions). The ratio by weight of hydrolyzable metal compound and alcohol preferably ranges from 0.1 to 0.25. The water content in the reaction mixture depends on the number of hydrolyzable groups in the metal compound and preferably is from 0.5 to 5, particularly from 1 to 3 moles of water per mole of hydrolyzable group.
The nanoscale metal oxide particles which are employed as catalysts according to the present invention are usually employed in an amount of 0.5 to 50% by wt., preferably 1 to 40% by wt. and particularly preferred 2 to 35% by wt., based on the total weight of the nanoscale particles and the species to be polymerized or crosslinked, respectively.
The manufacture of coatings according to the process of the present invention may, for example, be carried out by adding the nanoscale metal oxide, e.g., as such or in the form of a suspension prepared as described above, to the species to be polymerized or crosslinked, respectively. In order to adjust the rheological properties a solvent may be added to the resulting composition or solvent already present therein may be removed therefrom, respectively. Particularly in the case of the above described inorganic-organic species as species to be polymerized or crosslinked, respectively, alcohols which are liquid at room temperature and which may optionally contain ether groups (e.g. methanol, ethanol, propanols, butanols and butoxyethanol) are preferably used as the solvents to be added.
The resulting coating composition may then be applied by standard coating methods such as, e.g., dip coating, bar coating, brush coating, doctor blade coating, roll coating, spray coating and spin coating onto a suitable substrate which may have been subjected to a usual pretreatment in order to improve the adhesion. Preferably the substrate is a transparent substrate such as glass or a transparent plastic (such as polymethylmethacrylate).
The curing (organic polymerization or polyaddition, respectively and optionally, in addition thereto, a further condensation of precondensates derived from hydrolyzable silicone compounds) is carried out after an optional predrying operation at room temperature (in order to partially remove the solvent).
In the case of compositions for the manufacture of molded articles the volatile components (e.g. alcohols derived from hydrolysis, solvents for predispersing the particulate materials) are at least partially removed from the mixture (e.g. by distillation). Subsequently the concentrated mixture may, for example, be poured or injected, respectively into molds.
The organic polymerization or polyaddition, respectively of the polymerizable groups (e.g. methacrylate groups or epoxy groups) may take place thermally (preferably at temperatures ranging from 70 to 200xc2x0 C., particularly from 90 to 130xc2x0 C.) and/or be initiated by irradiation (preferably by means of UV light) in the presence of the nanoscale metal oxide particles employed according to the present invention. In the case of the photochemical polymerization it is particularly preferred, especially with inorganic-organic species as species to be polymerized or crosslinked, respectively, to carry out a thermal post-treatment at, for example, 90 to 130xc2x0 C. after the photochemical polymerization (i.e., a thermal post-condensation of condensable groups still present in said inorganic-organic species).
As already mentioned above the process according to the present invention is particularly suited for the manufacture of transparent molded articles and coatings for optical purposes but is not limited to said application. Furthermore it may be pointed out that the catalysts according to the present invention may, of course, serve a dual function (especially when used in the upper range of the quantitative ranges indicated), i.e., they may function not only as catalysts but also as (optionally reinforcing) filler for the polymerized matrix of organic or inorganic-organic species.
The nanoscale metal oxide particles employed in accordance with the present invention (particularly, those of SnO2, ZrO2, TiO2 and Al2O3) are also suitable for deblocking of blocked isocyanate functions. One may make use of this fact, for instance, in the preparation of polyurethanes and polyureas from blocked polyisocyanates and polyalcohols or polyamines.